This invention relates to skateboards, or more generally, to devices for human locomotion involving rolling or sliding, on which the rider stands with one foot ahead of the other and controls the direction of travel by articulation of the feet.
The classic skateboard design consists of a substantially rigid board elongated in the direction of travel having two wheel-sets mounted fore and aft to the underside of the board. These two wheel-sets, which each have two coaxial wheels spaced approximately 8 inches apart, are attached to the board using skateboard “trucks” which steer the wheels in response to left/right tilting of the board. The trucks also provide a spring-effect to resist tilting.
This method of steering has three deficiencies: limited steering travel, dynamic instability, and the inability to steer the two wheel-sets independently. The first two problems are inter-related. Large steering travel could be achieved with minimal tilting, but this would exacerbate the dynamic stability. At high speeds skateboards are prone to “death-wobble” in which the board steers left and right with increasing amplitude until the rider falls.
The third deficiency, lack of fore-aft steering independence, results from the use of a rigid board. In U.S. Pat. No. 4,082,306, Sheldon discloses this solution: cut the board in half and re-connect the fore and aft portions with a torsion bar. This allows the rider to tilt the front and rear trucks independently. While this provides additional mobility, for instance the ability to crab side-ways, it offers no improvement in steering travel or minimum turning radius.
In U.S. Pat. No. 4,955,626, Smith, Fisher and King describe a radically different type of skateboard. This invention is now a market success and is commonly referred to by its trade-name: “Snakeboard”. In this invention, the rider places his feet on two foot-platforms which are pivotably connected to a spacer element. The front and rear wheel-sets are positioned directly under the two foot pads, and steering is achieved by directly swiveling each foot pads about its vertical pivot axis. This arrangement provides independence of front and rear steering and a much greater range of steering angle than is practical with skateboard trucks. A key advantage of this invention is the ability to efficiently self-propel the board using a snake-like undulating motion. Since pushing off on the ground is unnecessary, the Snakeboard may be strapped to the rider's feet, which allows a range of jumps and tricks not possible with the conventional skateboard.
A significant problem with the Snakeboard is an inherent steering instability. This makes the board considerably more difficult to learn than the classic skateboard. Skateboards, snowboards, skis, surfboards and bicycles all have a tendency to steer in the direction of lean, which provides a natural self-righting effect. On a Snakeboard, however, the opposite is true.
The instability in this case is due to the outward (fore-aft) force on the two foot pads resulting from the rider's legs being spread apart. With weight balanced between toe and heel, there is no steering torque, but weighting the heels causes the outward force to be applied at the heels, resulting in a steering torque toward the toes. Similarly, weighting the toes results in a steering torque in the direction of the heels.
A second problem with the Snakeboard, as well as the classic skateboard is the sensitivity of the steering to road debris. If, for example the front right wheel hits a small pebble, the board will abruptly steer to the right.
A third problem is the trade-off between wheel diameter, height of the board and degree to which the board can be tilted. Ideally, the board should have large wheels, be as low as possible to the ground and be able to lean into a turn. With wheels mounted directly under foot, the Snakeboard cannot have large wheels and be low to the ground unless the wheels of each wheel-set are spaced very far apart. This solution adds excessive inertia about the steering axis.
The ability to lean or tilt the board provides for more natural and graceful motion and is a desirable feature for all skateboards. For this reason, the Snakeboard uses a spring-loaded tilt plate between each foot platform and wheel-set. As is also the case for the classic skateboard, additional height is required to allow the board to tilt without hitting the wheels.
Many of these problems are remedied by Barachet's two-wheel skateboard, disclosed in U.S. Pat. No. 5,160,155. This invention has a substantially rigid platform with a castering wheel in the front and a fixed wheel toward the rear. The rider stands with one foot ahead and the other behind the rear wheel. Steering of the front wheel results from tilting the board using the same principle which allows a bicycle to be ridden no-handed. While this device allows significant lean, has relatively large wheels, and is insensitive to road debris, it is less maneuverable and controllable than the Snakeboard, and is very inefficient at undulating self-propulsion. These deficiencies result from having indirect control over the front wheel, and no ability to steer the rear wheel.
With regard to skateboards for snow travel, there are several references in the prior art. In U.S. Pat. No. 5,613,695 Fu-Pin Yu describes a skateboard using Snakeboard-type steering with a single wide ski attached fore and aft in place of the two wheel-sets. This device would probably work reasonably well on fluffy snow, but on packed snow with the board tilted, turning the leading ski into the turn causes the leading edge to dig in to the snow, thus upsetting the rider. In U.S. Pat. No. 5,505,474 Hsiu-Ying Yeh presents a similar ski-board as a variation on his “folding skateboard”. In this case two skis are used under each foot instead of a single wide ski but again, the steering is unstable when the board is banked in a turn. Both Yu's and Yeh's inventions have a wide footprint and thus do not have the desired challenge of having to dynamically balance the board.